Typically, in the idle state, the target engine speed is set and an idle speed control procedure (hereinafter, ISC) is performed in correspondence with the target engine speed. More specifically, the amount of intake air fed to a combustion chamber is controlled by, for example, adjusting a throttle opening degree, in such a manner that the actual engine speed reaches the target engine speed.
Further, engines having a variable valve actuation mechanism that varies lift characteristics of an intake valve, such as opening timing, closing timing, open duration (lift duration angle), lift amount, and the relationship between the crank angle and the lift amount in an open state of the intake valve (lift profile), have been used. These engines improve engine output and maintain exhaust properties by changing the lift characteristics of the intake valve by means of the variable valve actuation mechanism (see, for example, Japanese Laid-Open Patent Publication No. 2001-263015).
However, if the ISC is carried out in these engines, which have the variable valve actuation mechanism, the following problem may occur.
In these engines, the intake valve functions as a restrictor regulating a flow of intake air. The extent of restriction is changed in correspondence with the lift characteristics of the intake valve, such as the lift amount and the open duration (the lift duration angle), which are changed by the variable valve actuation mechanism. This changes pumping loss caused by a throttle valve, leading to variation in response of the intake air amount with respect to changes of a throttle opening degree. The speed at which the engine speed is changed by the ISC is thus varied correspondingly.
Further, in the engines including the variable valve actuation mechanism, the amount of intake air fed to the combustion chamber is adjusted by varying the lift characteristics of the intake valve. This allows the engine to operate at a relatively low output (a relatively low air amount), compared to a case in which the intake air amount is reduced by decreasing the throttle opening degree.
More specifically, compared to the case in which the intake air amount is reduced by decreasing the throttle opening degree, the throttle opening degree is maintained at a relatively high level if the intake air amount is adjusted by changing the lift characteristics of the intake valve. This raises the pressure of the intake air in the intake passage upstream from the intake valve. Therefore, if the amount of intake air fed to the combustion chamber, or the pressure acting in the combustion chamber when an intake stroke is complete, is equal in the two cases, the pressure acting in the combustion chamber before completion of the intake stroke is relatively high in the case in which the intake air amount is controlled through changes of the lift characteristics of the intake valve, compared to the case in which such control is performed through adjustment of the throttle valve. Accordingly, the pumping loss becomes relatively low if the intake air amount is controlled through changes of the lift characteristics of the intake valve, compared to the other case.
However, in controlling of the lift characteristics by the variable valve actuation mechanism, the throttle opening degree (the intake air amount) required for adjusting the actual engine speed to the target engine speed changes in correspondence with changes of the pumping loss. This may vary the speed at which the engine speed is changed by the ISC.
As has been described, the conventional the ISC is not adapted to changes of the engine properties, which are brought about by changes of the lift characteristics of the intake valve. This lowers the control reliability of the engine speed.